Method of producing sintered ferrous materials

ABSTRACT

THE USES OF SINTERED FERROUS MATERIALS FOR IMPORTANT STRUCTURAL MACHINE PARTS ARE CONSIDERABLY LIMITED BECAUSE OF SUBSTANTIALLY INFERIOR MECHANICAL PROPERTIES IN USUAL FORGINGS AND CASTINGS OF THE SAME COMPOSITION IN SPITE OF THE FACT THAT SINTERING MATERIALS CAN BE PRODUCED AT RELATIVELY LOW COST AND THEY ELIMINATE MACHINING DUE TO THE CLOSE TOLERANCES OBTAINABLE. IN ORDER TO OVERCOME THE ABOVE-MENTIONED FAULTS AND TO ATTAIN SINTERED FERROUS MATERIALS WITH HIGH DENSITY, HIGH STRENGTH AND EXCELLENT DUCTILITY, THERE IS PROVIDED A METHOD OF LIQUID PHASE SINTERING OF FERROUS MATERIALS IN WHICH FE-SI ALLOY POWDERS WITH MORE THAN 7 WEIGHT PERCENT OF SI, AND THE REMAINDER FE ARE ADDED TO IRON POWDERS AT THE RATE OF 0.3-10 WEIGHT PERCENT OF SI.

United States Patent Oflice Int. Cl. 1322f 1/00 US. Cl. 75214 4 ClaimsABSTRACT OF THE DISCLOSURE The uses of sintered ferrous materials forimportant structural machine parts are considerably limited because ofsubstantially inferior mechanical properties in usual forgings andcastings of the same composition in spite of the fact that sinteringmaterials can be produced at relatively low cost and they eliminatemachining due to the close tolerances obtainable.

In order to overcome the above-mentioned faults and to attain sinteredferrous materials with high density, high strength and excellentductility, there is prow'ded a method of liquid phase sintering offerrous materials in which Fe-Si alloy powders with more than 7 weightpercent of Si, and the remainder Fe are added to iron powders at therate of 03-10 weight percent of Si.

BACKGROUND OF THE INVENTION The present invention relates to a method ofproducing sintered ferrous materials.

Owing to the advantages that sintered materials can be produced at arelatively low cost and scarcely need to be machined because of the gooddimensional accuracy of their products, sintered materials have beenused as structural machine parts. But sintered materials inevitablybecome porous through their production method, and so they are inferiorin mechanical properties to castings and forgings of the samecomposition. Particularly, in sintered ferrous materials where thevariation of dimensions between before and after sintering is verysmall, such a tendency is so remarkable that the sintered ferrousmaterials in the present state are hardly applicable for importantstructural parts or for heavily loaded usages.

Efforts have been made to produce sintered materials having densitiesclose to the theoretical value. The object of such effort was toremarkably enlarge the application of such sintered materials. A methodhas been developed for reducing the porosity of such sintered materialsas much as possible and for rasin-g the density and consequently thestrength of such materials. This prior attempt involves increasing thecompacting pressure. For instance, in the case of reduced iron powder,the density of green compacts amounts to about 78% of the real densityof iron at compacting pressure of 4 ton/cmF, and to 90% at 8 ton/cm.Like this, the green compacts and consequently more dense sinteredmaterials are attained by increasing the compacting pressure. On theother hand, this method leads to a short die life due to an increasingdie wear and requires a large capacity press, so that the adoption ofthis method is difiicult for practical reasons. Therefore, compactingpressure is limited to 4-6 ton/cm. at most, and then, the density ofgreen compacts obtained in such a way is in general only on the order of6.0-6.6 gr./cm. Accordingly, when such green compacts are sintered, itis considerably more difiicult to obtain dense sintered materials,because the progress of sintering is very low and the materials scarcelyshrink owing to the solid phase sintering,

Moreover, as a manufacturing method for sintered ferrous materials of ahigh density, such a method as (l) Patented Sept. 5, 1972 multi-pressingand multi-sintering, (2) liquid phase sintering and (3) infiltration hasbeen usually employed. However, these methods have the following faults:

(l) The multi-cornpacting and multi-sintering method is such that thecompaction and sintering are repeated more than several times so thatthe cost of such a sintered material becomes inevitably expensive.

(2) The liquid phase sintering is a useful method as a means ofattaining dense sintered products. For sintering of iron powders it hasbeen intended to accelerate the sintering rate by the addition of boronor phosphorous to the iron powder in the form of powdered ferro-boron orferro phosphorous, each of which yields the liquid phase at thesintering temperature employed. In this case, the greater the quantityof the additive, the better the densification attained, and so densesintered products are attained. On the contrary, the addition of above0.8% P and above 0.2% B causes hard and brittle eutectoid constituent toremain at the grain boundaries and as a result, to reduce considerablythe ductility of the sintered materials, consequently, this method isnot suitable for practical uses. Besides, a liquid phase sintering bythe addition of Cu has been carried out, but this method is notpractical because the intrinsic properties of Cu itself make a sinterediron brittle.

(3) Infiltration is the method of filling molten Cu into the pores ofsintered iron by the use of capillary phenomenon thereby making thesintered materials dense. However, this method exceedingly limits theductility of sintered products as in the case of liquid phase sinteringby the addition of Cu.

SUMMARY OF THE INVENTION The principal object of the present inventionis to obviate the above-mentioned faults of the usual sintering methodsand to produce sintered irons having high density, high strength andexcellent ductility.

In accordance with the invention there is proposed a method of producingsintered ferrous materials comprising a sintering operation with theaddition of Fe-Si alloy powders comprising more than seven weightpercent Si the balance of said powder being Fe, to iron powders with anaddition ratio to said powders of 03-10 weight percent of Si.

That is, the method of the present invention consists in adding to ironpowders the Fe-Si alloy which is composed of more than 7 percent byweight of Si with the balance thereof being Fe, in place of boron orphosphorous since these letters can not be added thereto in suchquantities to make a satisfactory improvement in the sinteringcharacteristics of the iron powder. The solid solubility of Si into Feis so large that quantities of Si up to 10% can be added. Consequently,the rate of sintering can be sufiiciently improved, while the matrix canalso be strengthened by Si so that it becomes easy to get sinteredferrous materials of high density, high strength and excellentductility.

The effects of Fe-Si alloy in the present invention will be explained asfollows: The alloys of below 7 weight percent Si, are not practicalbecause their liquid phase may not be formed below 1400 C. On the otherhand, Fe-Si alloys containing above 7 weight percent Si may yield aliquid phase below 1400 C. so that the liquid phase sintering can becarried out practically without question. In the above-mentionedcomposition range of Fe-Si alloys, alloys containing above 17.5 Weightpercent Si are especially favourable, since they form a liquid phase atl2001220 C. or higher. As will be shown in the following executedexamples, the addition of these alloys are capable of acceleratingmarkedly the progress of sintering at just above the temperature ofliquid phase formation, that is, even at 1225 C. Therefore, it is thefeature of the present invention to be capable of sintering attemperatures near 1130-l220 C., which are generally adopted as thesintering temperatures of iron. In this 4 In short, according to thepresent invention, sintered ferrous materials having high strength andexcellent ductility can easily be produced with usual forming pressure,so that the method of the present invention is exceedingly case, as thequantities of Fe-Si alloy powders added, the 5 useful.

TABLE 1 I 0.2% proof Tensile Reduction Fe-Si alloy Sintering stressstrength Elongation of area (Si percent) time On.) (kgJmmJ) (kg/mm?)(percent) (percent) The method of the present 2 3 21. 6 40. 5 19. 2 21.3 invention 5 22. 3 42. 8 26.0 33. 4 1 29. 2 46.8 25.2 31. 1

The usual method 1 8.6 17. 9 19. 2 14. 6 5 10. 19.4 15.0 14.8

NOTE.-Sintering temperature=1,275 0.; compacting pressure=5 ton/cmfl.

TABLE 2 0.2% proof Tensile Reduction Fe-Si alloy Sintering stressstrength Elongation of area (Si percent) time (hr.) (kgJmmJ) (kg/mmfi)(percent) (percent) The method of the 1 35. 8 50. 6 18. 8 23. presentinvention 4 3 37.8 54. 1 23. 2 33.

.- The usual method 3'2 Z NorE.Siutering temperaturc=1,225 0.;compacting pressure=5 ton/cmfl.

addition of 0.3-10 weight percent Si to iron powders is desirable. Thereason why this composition range is determined is as follows; theaddition of less than 0.3 weight percent Si hardly improves the strengthof sintered materials and the addition of more than 10 weight percent Siabruptly reduces their mechanical properties.

DESCRIPTION OF A PREFERRED EMBODIMENT The examples executed according tothe present invention will be explained as follows: In Table 1, themechanical properties of the sintered irons sintered at 1275 C. arecompared between the method of the present invention with differentadditions of Fe-Si alloys and the usual method. In Table 2, themechanical properties of the sintered irons sintered at 1225 C. arecompared between the method of the present invention and the usualmethod. From these executed examples it will clearly be seen that themethod of the present invention permits the attainment of sintered ironswith strength and excellent ductility.

Moreover, it goes without saying that the method of the presentinvention gives full play to the eifect in the case of adding suchalloying elements to iron as C, Mn, Cr, Ni, Cu, Co, W, Mo, Cb, Ti andthe like for the purpose of strengthening of the matrix.

What is claimed is:

1. Process for production of high strength and density ferrous materialswhich comprises admixing ferrosilicon alloy powder with iron powder inan amount sufiicient to yield a mixture of iron powder and ferrosiliconalloy powder having an amount of silicon therein from about 0.3 to about10 percent by weight, said ferrosilicon alloy powder consistingessentially of silicon in an amount from just more than seven percent byweight of said alloy to an amount adapted to maintain said alloy inliquid phase at temperatures from about 1225 C. to about 1275 C., theremainder of said alloy being iron; whereby a ferrous material isproduced having a tensile strength of from about 23 kg./mm. to about 64kg./mm. and a 0.2% proof stress of from about 12 to about 55 kg./rnm.

2. The process of claim 1, in which the sintering is carried out for aperiod of from about one to about ten hours.

3. Process according to claim 1, in which the compacting is carried outat about 5 ton/cm.

4. Process according to claim 3, in which the sintering is carried outfor a period of from about one to about five hours.

(References on following page) 5 6 References Cited R. L. Sands et al.:Powder Metallergy, p. 210, George UNITED ST P Newnes Limited (1966),TN695S2.

2,226,520 11/1939 Lenel 75-400 CARL D. QUARFORTH, Primary Examiner OTHERREFERENCES 5 R. E. SCI-IAFER, Asslstant Exammer Constitution of BinaryAlloys, Mat Hansen, 2nd edi- U.S. Cl. X.R. tion, McGraw-Hill, 1958,TA490, H27aE, pp. 711717. 75-200, 227

